Cold cathode ionization vacuum gauges (CCIGs) are well known. Three commonly known CCIGs include normal (noninverted) magnetron type gauges, inverted magnetron type gauges, and Philips (or Penning) gauges. All of these types of gauges have at least two electrodes (i.e., an anode and a cathode) in an evacuated non-magnetic envelope which is connected to the vacuum to be measured. A high DC voltage potential difference is applied between the anode electrode and the cathode electrode to create an electronic field between the electrodes. A magnetic field is applied along the axis of the electrodes perpendicular to the electric field in order to lengthen free electron paths to sustain a pure electron plasma in which the electrons collide with molecules and atoms to create ions. The ions move to the cathode electrode to maintain the discharge current at a steady state value which is a function of pressure.
A CCIG provides an indirect measurement of vacuum system total pressure by first ionizing gas molecules and atoms inside its vacuum gauge envelope and then measuring the resulting ion current. The measured ion current is directly related to the gas density and gas total pressure inside the gauge envelope, i.e., as the pressure inside the vacuum system decreases, the measured ion current decreases. Gas specific calibration curves provide the ability to calculate total pressures based on ion current measurements.
The CCIG described herein relies on the inverted magnetron principle. The gauge is of cylindrical symmetry. A large voltage potential gradient (i.e., radial electric field) between the anode pin (located at the axis) and the cathode cylindrical envelope provides energy to the electrons for the ionization events to occur. A crossed axial magnetic field provides the long electron trajectory path length required to maintain a pure electron plasma inside the envelope. The discharge current is the measured quantity that is proportional to the pressure in the system.
The discharge is established through an avalanche ionization process that generally starts with a single electron being released into the ionization volume of the gauge. The process responsible for releasing an electron can include a field emission event or a cosmic ray ionization process. The avalanche process relies on the long path length for the electron trajectories that leads to many ionization processes per electron. Each ionization process releases an ion as well as an additional electron that is added into the discharge. As the ions collide with the cathode internal walls, additional electrons are also released into the discharge, thereby contributing to the total charge. As a result of the crossed electric and magnetic fields, a pure electron plasma builds as a sheath around the anode. The electron density is predominantly independent of pressure. Ionization of neutral gas molecules takes place predominantly inside the pressure invariant electron sheath. All ions produced are directed to the cathode by the electric field and with little influence of the magnetic field. The resulting ion current is simply related to the electron density and the total pressure of gas inside the sensor.
The double inverted magnetron design of U.S. patent application Ser. No. 14/500,820, U.S. Publication No. 2015/0091579 to Brucker, et al., shown in FIG. 1A, includes two magnets 115a, 115b held together in a magnet assembly, the two magnets having their magnetic poles opposed to one another. The double inverted magnetron features some of the largest magnetic fields, and as a result provides the largest gauge sensitivities available. Large gauge sensitivities are required to be able to read reliable pressures at ultrahigh vacuum (UHV) levels (i.e., pressures less than about 10−9 Torr and as low as 10−11 Torr). U.S. patent application Ser. No. 14/500,820 is incorporated in its entirety by reference.
In CCIGs of the inverted magnetron type, it is possible for a small leakage current to flow directly from the anode 110 to the cathode 120 via the internal surfaces of the gauge, and it is known that the presence of a so-called “guard ring” can collect this leakage current and thereby prevent it from reaching the cathode electrode and being detected by the gauge itself. To perform this function, the guard ring is electrically isolated from the cathode electrode and normally held at a small positive voltage potential difference relative to the cathode electrode.
As shown in FIGS. 1A, 1B, and 1C, a CCIG 100 includes a feedthrough 101 that includes a guard ring connection 102 that provides electrical connection to a guard ring electrode 140 described below. Inside the guard ring connection 102, an anode guard ring insulator 106 provides electrical insulation around an anode connection 110a to an extended anode electrode 110. The guard ring electrode 140 is connected to a starter device 150, which is described below. The guard ring connection 102 is connected by a cathode-guard ring insulator 103 to a weld surface 104, which is seam welded to a monolithic flange assembly 105. As shown in FIGS. 1A and 1B, the monolithic flange assembly 105 includes outer flange 105a and inner flange 105b. The inner flange 105b encloses a cathode electrode 120 surrounding the anode electrode 110 along its length and forming a discharge space 130 between the anode electrode 110 and the cathode electrode 120. A baffle, shown in FIG. 1A as two partitions 170 and 180 having apertures 175 and 185, respectively, is connected to the cathode electrode 120.
As discussed above, a crossed axial magnetic field provides the electron trajectory path length required to maintain a discharge inside the discharge space 130. The magnetic field is created by magnet assembly 115, shown in FIGS. 1A and 1B. The magnet assembly 115 includes a ferromagnetic spacer 114. The magnet assembly 115 can also include an aluminum (or other non-magnetic material) spacer 113 at the end of the magnet assembly closest to the guard ring 140 to adjust the location of the electrical discharge away from the guard ring 140.
The electrically conductive guard ring electrode 140 is interposed between the cathode electrode 120 and the anode electrode 110 about a base of the anode electrode 110 to collect leakage electrical current that would otherwise tend to flow between the anode electrode 110 and the cathode electrode 120 if electrically conductive deposits accumulate over time on surfaces of the cathode-guard ring insulator 103 exposed to the discharge space 130 during operation of the vacuum gauge 100.
A discharge starter device 150 is disposed over and electrically connected with the guard ring electrode 140. As shown in FIG. 1B, the starter device 150 has a plurality of tips 160 (3 tips are shown in the cross-section cylindrically symmetrical view shown in FIG. 1B) directed toward the anode 110 and forming a gap between the tips 160 and the anode 110. The gap between the tips and the anode can be in a range of about 500 μm to about 2500 μm. The gap is configured such that the field emission current during normal operation is in a range of about 1 picoamp (pA) to about 10 pA when a voltage potential difference between the starter device 150 and the anode 110 is established. The field emission current amplitude is dependent on several parameters, such as the voltage potential difference, the size of the gap, the number of points on the starter device, and the type of material that the starter device is made of The voltage potential difference between the starter device and the anode, during operation of the CCIG, can be in a range of about 0.4 kilovolt (kV) to about 6 kV, for example, approximately 3.5 kV. This voltage potential difference produces electrons by field emission from the sharp tips 160 to the anode, thereby seeding some electrons into the discharge volume 130 to trigger the avalanche process that is responsible for building up the discharge. Optionally, the voltage potential difference between the starter device and the anode can be configured to be increased from about 3.5 kV to about 5 kV during startup of the gauge, in order to increase the field emission current by increasing the high voltage supply bias to the anode electrode momentarily, until a discharge is detected by a sudden increase in the discharge current.
As shown in the electronic controller of FIG. 1C, a limiting resistor 410 is placed between the anode electrode 110 and the high voltage power supply 430 (HVPS). The role of the limiting resistor 410 is to put an upper limit to the amount of discharge current that can flow through the discharge volume 130 and to extend the lifetime of the vacuum gauge. As a result of the limiting resistor 410, the actual high voltage bias present at the anode electrode 110 and measured by voltmeter 420 is generally smaller than the voltage delivered by the HVPS 430. In fact, the anode voltage decreases as the ion current increases with pressure, even though the output of the HVPS 430 remains constant at all pressures. In the vacuum gauge described herein, a 25 Megaohm (MΩ) limiting resistor 410 was selected to provide several advantages: 1. a safety limit to the amount of current the HVPS can deliver to an individual in case of accidental contact with internal HVPS components, 2. the choice of resistor moves pressure curve discontinuities into the higher pressure range above 1×10−6 Torr, and 3. an upper limit for the discharge current of 125 μA when the anode voltage is set to 3.5 kV. The processor 490 of the CCIG controller ensures that the output of the HVPS 430 is constant over the entire pressure range while the processor continuously measures the anode voltage V with voltmeter 420 and discharge current ID with ammeter 460 to calculate discharge impedance Z as a function of pressure. The processor also measures guard ring current with a meter 470 to monitor current leakage. With this circuit configuration, two independent current loops assure that leakage currents at the anode feedthrough do not cause any inaccuracies in the pressure measurement that depends on discharge current impedance measurements.
CCIGs are typically limited to operation in low pressure ranges below 10−2 Torr. To measure pressure over ranges that extend as high as atmospheric pressure (760 Torr), they may be combined with pressure gauges using different technologies, such as thermal conductivity or diaphragm gauges.